man-pages/man2/select_tut.2

.\" This manpage is copyright (C) 2001 Paul Sheer.
.\"
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.\" very minor changes, aeb
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.\" Modified 5 June 2002, Michael Kerrisk <mtk.manpages@gmail.com>
.\" 2006-05-13, mtk, removed much material that is redundant with select.2
.\"             various other changes
.\"
.TH SELECT_TUT 2 2006-05-13 "Linux" "Linux Programmer's Manual"
.SH NAME
select, pselect, FD_CLR, FD_ISSET, FD_SET, FD_ZERO \-
synchronous I/O multiplexing
.SH SYNOPSIS
.nf
/* According to POSIX.1-2001 */
.br
.B #include <sys/select.h>
.sp
/* According to earlier standards */
.br
.B #include <sys/time.h>
.br
.B #include <sys/types.h>
.br
.B #include <unistd.h>
.sp
.BI "int select(int " nfds ", fd_set *" readfds ", fd_set *" writefds ,
.BI "           fd_set *" exceptfds ", struct timeval *" timeout );
.sp
.BI "void FD_CLR(int " fd ", fd_set *" set );
.br
.BI "int  FD_ISSET(int " fd ", fd_set *" set );
.br
.BI "void FD_SET(int " fd ", fd_set *" set );
.br
.BI "void FD_ZERO(fd_set *" set );
.sp
.B #include <sys/select.h>
.sp
.BI "int pselect(int " nfds ", fd_set *" readfds ", fd_set *" writefds ,
.BI "            fd_set *" exceptfds ", const struct timespec *" timeout ,
.BI "            const sigset_t *" sigmask );
.fi
.sp
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.sp
.BR pselect ():
_POSIX_C_SOURCE\ >=\ 200112L || _XOPEN_SOURCE\ >=\ 600
.SH DESCRIPTION
.BR select ()
(or
.BR pselect ())
is the pivot function of
most C programs that
handle more than one simultaneous file descriptor (or socket handle)
in an efficient
manner.
Its principal arguments are three arrays of file descriptors:
\fIreadfds\fP, \fIwritefds\fP, and \fIexceptfds\fP.
The way that
.BR select ()
is usually used is to block while waiting for a "change of
status" on one or more of the file descriptors.
A "change of status" is
when more characters become available from the file descriptor, \fIor\fP
when space becomes available within the kernel's internal buffers for
more to be written to the file descriptor, \fIor\fP when a file
descriptor goes into error (in the case of a socket or pipe this is
when the other end of the connection is closed).

In summary,
.BR select ()
just watches multiple file descriptors,
and is the standard Unix call to do so.

The arrays of file descriptors are called \fIfile descriptor sets\fP.
Each set is declared as type \fBfd_set\fP, and its contents can be
altered with the macros
.BR FD_CLR (),
.BR FD_ISSET (),
.BR FD_SET (),
and
.BR FD_ZERO ().
.BR FD_ZERO ()
is usually the first function to be used on
a newly declared set.
Thereafter, the individual file descriptors that
you are interested in can be added one by one with
.BR FD_SET ().
.BR select ()
modifies the contents of the sets according to the rules
described below; after calling
.BR select ()
you can test if your file
descriptor is still present in the set with the
.BR FD_ISSET ()
macro.
.BR FD_ISSET ()
returns non-zero if the descriptor is present and zero if
it is not.
.BR FD_CLR ()
removes a file descriptor from the set.
.SS Arguments
.TP
\fIreadfds\fP
This set is watched to see if data is available for reading from any of
its file descriptors.
After
.BR select ()
has returned, \fIreadfds\fP will be
cleared of all file descriptors except for those file descriptors that
are immediately available for reading with a
.BR recv (2)
(for sockets) or
.BR read (2)
(for pipes, files, and sockets) call.
.TP
\fIwritefds\fP
This set is watched to see if there is space to write data to any of
its file descriptors.
After
.BR select ()
has returned, \fIwritefds\fP will be
cleared of all file descriptors except for those file descriptors that
are immediately available for writing with a
.BR send (2)
(for sockets) or
.BR write (2)
(for pipes, files, and sockets) call.
.TP
\fIexceptfds\fP
This set is watched for exceptions or errors on any of the file
descriptors.
However, that is actually just a rumor.
How you use
\fIexceptfds\fP is to watch for \fIout\-of\-band\fP (OOB) data.
OOB data
is data sent on a socket using the \fBMSG_OOB\fP flag, and hence
\fIexceptfds\fP only really applies to sockets.
See
.BR recv (2)
and
.BR send (2)
about this.
After
.BR select ()
has returned,
\fIexceptfds\fP will be cleared of all file descriptors except for those
descriptors that are available for reading OOB data.
You can only ever
read one byte of OOB data though (which is done with
.BR recv (2)),
and
writing OOB data (done with
.BR send (2))
can be done at any time and will
not block.
Hence there is no need for a fourth set to check if a socket
is available for writing OOB data.
.TP
\fInfds\fP
This is an integer one more than the maximum of any file descriptor in
any of the sets.
In other words, while you are busy adding file descriptors
to your sets, you must calculate the maximum integer value of all of
them, then increment this value by one, and then pass this as \fInfds\fP to
.BR select ().
.TP
\fIutimeout\fP
.RS
This is the longest time
.BR select ()
must wait before returning, even
if nothing interesting happened.
If this value is passed as NULL,
then
.BR select ()
blocks indefinitely waiting for an event.
\fIutimeout\fP can be set to zero seconds, which causes
.BR select ()
to
return immediately.
The structure \fIstruct timeval\fP is defined as,
.PP
.nf
struct timeval {
    time_t tv_sec;    /* seconds */
    long tv_usec;     /* microseconds */
};
.fi
.RE
.TP
\fIntimeout\fP
.RS
This argument has the same meaning as \fIutimeout\fP but \fIstruct timespec\fP
has nanosecond precision as follows,
.PP
.nf
struct timespec {
    long tv_sec;    /* seconds */
    long tv_nsec;   /* nanoseconds */
};
.fi
.RE
.TP
\fIsigmask\fP
This argument holds a set of signals to allow while performing a
.BR pselect ()
call (see
.BR sigaddset (3)
and
.BR sigprocmask (2)).
It can be passed
as NULL, in which case it does not modify the set of allowed signals on
entry and exit to the function.
It will then behave just like
.BR select ().
.SS Combining Signal and Data Events
.BR pselect ()
must be used if you are waiting for a signal as well as
data from a file descriptor.
Programs that receive signals as events
normally use the signal handler only to raise a global flag.
The global
flag will indicate that the event must be processed in the main loop of
the program.
A signal will cause the
.BR select ()
(or
.BR pselect ())
call to return with \fIerrno\fP set to \fBEINTR\fP.
This behavior is
essential so that signals can be processed in the main loop of the
program, otherwise
.BR select ()
would block indefinitely.
Now, somewhere
in the main loop will be a conditional to check the global flag.
So we
must ask: what if a signal arrives after the conditional, but before the
.BR select ()
call?
The answer is that
.BR select ()
would block
indefinitely, even though an event is actually pending.
This race
condition is solved by the
.BR pselect ()
call.
This call can be used to
mask out signals that are not to be received except within the
.BR pselect ()
call.
For instance, let us say that the event in question
was the exit of a child process.
Before the start of the main loop, we
would block \fBSIGCHLD\fP using
.BR sigprocmask (2).
Our
.BR pselect ()
call would enable \fBSIGCHLD\fP by using the virgin signal mask.
Our
program would look like:
.PP
.nf
int child_events = 0;

void
child_sig_handler(int x)
{
    child_events++;
    signal(SIGCHLD, child_sig_handler);
}

int
main(int argc, char **argv)
{
    sigset_t sigmask, orig_sigmask;

    sigemptyset(&sigmask);
    sigaddset(&sigmask, SIGCHLD);
    sigprocmask(SIG_BLOCK, &sigmask, &orig_sigmask);

    signal(SIGCHLD, child_sig_handler);

    for (;;) { /* main loop */
        for (; child_events > 0; child_events\-\-) {
            /* do event work here */
        }
        r = pselect(nfds, &rd, &wr, &er, 0, &orig_sigmask);

        /* main body of program */
    }
}
.fi
.SS Practical
So what is the point of
.BR select ()?
Can't I just read and write to my
descriptors whenever I want?
The point of
.BR select ()
is that it watches
multiple descriptors at the same time and properly puts the process to
sleep if there is no activity.
It does this while enabling you to handle
multiple simultaneous pipes and sockets.
Unix programmers often find
themselves in a position where they have to handle I/O from more than one
file descriptor where the data flow may be intermittent.
If you were to
merely create a sequence of
.BR read (2)
and
.BR write (2)
calls, you would
find that one of your calls may block waiting for data from/to a file
descriptor, while another file descriptor is unused though available
for data.
.BR select ()
efficiently copes with this situation.

A simple example of the use of
.BR select ()
can be found in the
.BR select (2)
manual page.
.SS Select Law
Many people who try to use
.BR select ()
come across behavior that is
difficult to understand and produces non-portable or borderline
results.
For instance, the above program is carefully written not to
block at any point, even though it does not set its file descriptors to
non-blocking mode at all (see
.BR ioctl (2)).
It is easy to introduce
subtle errors that will remove the advantage of using
.BR select (),
hence I will present a list of essentials to watch for when using the
.BR select ()
call.
.TP
\fB1.\fP
You should always try to use
.BR select ()
without a timeout.
Your program
should have nothing to do if there is no data available.
Code that
depends on timeouts is not usually portable and is difficult to debug.
.TP
\fB2.\fP
The value \fInfds\fP must be properly calculated for efficiency as
explained above.
.TP
\fB3.\fP
No file descriptor must be added to any set if you do not intend
to check its result after the
.BR select ()
call, and respond
appropriately.
See next rule.
.TP
\fB4.\fP
After
.BR select ()
returns, all file descriptors in all sets
should be checked to see if they are ready.
.\" mtk, May 2006: the following isn't really true.
.\" Any file descriptor that is available
.\" for writing \fImust\fP be written to, and any file descriptor
.\" available for reading \fImust\fP be read, etc.
.TP
\fB5.\fP
The functions
.BR read (2),
.BR recv (2),
.BR write (2),
and
.BR send (2)
do \fInot\fP necessarily read/write the full amount of data
that you have requested.
If they do read/write the full amount, its
because you have a low traffic load and a fast stream.
This is not
always going to be the case.
You should cope with the case of your
functions only managing to send or receive a single byte.
.TP
\fB6.\fP
Never read/write only in single bytes at a time unless your are really
sure that you have a small amount of data to process.
It is extremely
inefficient not to read/write as much data as you can buffer each time.
The buffers in the example above are 1024 bytes although they could
easily be made larger.
.TP
\fB7.\fP
The functions
.BR read (2),
.BR recv (2),
.BR write (2),
and
.BR send (2)
as well as the
.BR select ()
call can return \-1 with
.I errno
set to \fBEINTR\fP,
or with
.I errno
set to \fBEAGAIN\fP (\fBEWOULDBLOCK\fP).
These results must be properly managed (not done properly
above).
If your program is not going to receive any signals then
it is unlikely you will get \fBEINTR\fP.
If your program does not
set non-blocking I/O, you will not get \fBEAGAIN\fP.
Nonetheless
you should still cope with these errors for completeness.
.TP
\fB8.\fP
Never call
.BR read (2),
.BR recv (2),
.BR write (2),
or
.BR send (2)
with a buffer length of zero.
.TP
\fB9.\fP
If the functions
.BR read (2),
.BR recv (2),
.BR write (2),
and
.BR send (2)
fail
with errors other than those listed in \fB7.\fP,
or one of the input functions returns 0, indicating end of file,
then you should \fInot\fP pass that descriptor to
.BR select ()
again.
In the above example,
I close the descriptor immediately, and then set it to \-1
to prevent it being included in a set.
.TP
\fB10.\fP
The timeout value must be initialized with each new call to
.BR select (),
since some operating systems modify the structure.
.BR pselect ()
however does not modify its timeout structure.
.TP
\fB11.\fP
I have heard that the Windows socket layer does not cope with OOB data
properly.
It also does not cope with
.BR select ()
calls when no file
descriptors are set at all.
Having no file descriptors set is a useful
way to sleep the process with sub-second precision by using the timeout.
(See further on.)
.SS Usleep Emulation
On systems that do not have a
.BR usleep (3)
function, you can call
.BR select ()
with a finite timeout and no file descriptors as
follows:
.PP
.nf
    struct timeval tv;
    tv.tv_sec = 0;
    tv.tv_usec = 200000;  /* 0.2 seconds */
    select(0, NULL, NULL, NULL, &tv);
.fi
.PP
This is only guaranteed to work on Unix systems, however.
.SH RETURN VALUE
On success,
.BR select ()
returns the total number of file descriptors
still present in the file descriptor sets.

If
.BR select ()
timed out, then
the return value will be zero.
The file descriptors set should be all
empty (but may not be on some systems).

A return value of \-1 indicates an error, with \fIerrno\fP being
set appropriately.
In the case of an error, the contents of the returned sets and
the \fIstruct timeout\fP contents are undefined and should not be used.
.BR pselect ()
however never modifies \fIntimeout\fP.
.SH NOTES
Generally speaking, all operating systems that support sockets, also
support
.BR select ().
Many types of programs become
extremely complicated without the use of
.BR select ().
.BR select ()
can be used to solve
many problems in a portable and efficient way that naive programmers try
to solve in a more complicated manner using
threads, forking, IPCs, signals, memory sharing, and so on.
.PP
The
.BR poll (2)
system call has the same functionality as
.BR select (),
and is somewhat more efficient when monitoring sparse
file descriptor sets.
It is nowadays widely available,
but historically was less portable than
.BR select ().
.PP
The Linux-specific
.BR epoll (7)
API provides an interface that is more efficient than
.BR select (2)
and
.BR poll (2)
when monitoring large numbers of file descriptors.
.SH EXAMPLE
Here is an example that better demonstrates the true utility of
.BR select ().
The listing below is a TCP forwarding program that forwards
from one TCP port to another.
.PP
.nf
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/time.h>
#include <sys/types.h>
#include <string.h>
#include <signal.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <errno.h>

static int forward_port;

#undef max
#define max(x,y) ((x) > (y) ? (x) : (y))

static int
listen_socket(int listen_port)
{
    struct sockaddr_in a;
    int s;
    int yes;
    if ((s = socket(AF_INET, SOCK_STREAM, 0)) < 0) {
        perror("socket");
        return \-1;
    }
    yes = 1;
    if (setsockopt(s, SOL_SOCKET, SO_REUSEADDR,
            (char *) &yes, sizeof(yes)) < 0) {
        perror("setsockopt");
        close(s);
        return \-1;
    }
    memset(&a, 0, sizeof(a));
    a.sin_port = htons(listen_port);
    a.sin_family = AF_INET;
    if (bind(s, (struct sockaddr *) &a, sizeof(a)) < 0) {
        perror("bind");
        close(s);
        return \-1;
    }
    printf("accepting connections on port %d\\n", listen_port);
    listen(s, 10);
    return s;
}

static int
connect_socket(int connect_port, char *address)
{
    struct sockaddr_in a;
    int s;
    if ((s = socket(AF_INET, SOCK_STREAM, 0)) < 0) {
        perror("socket");
        close(s);
        return \-1;
    }

    memset(&a, 0, sizeof(a));
    a.sin_port = htons(connect_port);
    a.sin_family = AF_INET;

    if (!inet_aton(address, (struct in_addr *) &a.sin_addr.s_addr)) {
        perror("bad IP address format");
        close(s);
        return \-1;
    }

    if (connect(s, (struct sockaddr *) &a, sizeof(a)) < 0) {
        perror("connect()");
        shutdown(s, SHUT_RDWR);
        close(s);
        return \-1;
    }
    return s;
}

#define SHUT_FD1 {                      \\
        if (fd1 >= 0) {                 \\
            shutdown(fd1, SHUT_RDWR);   \\
            close(fd1);                 \\
            fd1 = \-1;                   \\
        }                               \\
    }

#define SHUT_FD2 {                      \\
        if (fd2 >= 0) {                 \\
            shutdown(fd2, SHUT_RDWR);   \\
            close(fd2);                 \\
            fd2 = \-1;                   \\
        }                               \\
    }

#define BUF_SIZE 1024

int
main(int argc, char **argv)
{
    int h;
    int fd1 = \-1, fd2 = \-1;
    char buf1[BUF_SIZE], buf2[BUF_SIZE];
    int buf1_avail, buf1_written;
    int buf2_avail, buf2_written;

    if (argc != 4) {
        fprintf(stderr,
                 "Usage\\n\\tfwd <listen-port> "
                 "<forward-to-port> <forward-to-ip-address>\\n");
        exit(EXIT_FAILURE);
    }

    signal(SIGPIPE, SIG_IGN);

    forward_port = atoi(argv[2]);

    h = listen_socket(atoi(argv[1]));
    if (h < 0)
        exit(EXIT_FAILURE);

    for (;;) {
        int r, nfds = 0;
        fd_set rd, wr, er;
        FD_ZERO(&rd);
        FD_ZERO(&wr);
        FD_ZERO(&er);
        FD_SET(h, &rd);
        nfds = max(nfds, h);
        if (fd1 > 0 && buf1_avail < BUF_SIZE) {
            FD_SET(fd1, &rd);
            nfds = max(nfds, fd1);
        }
        if (fd2 > 0 && buf2_avail < BUF_SIZE) {
            FD_SET(fd2, &rd);
            nfds = max(nfds, fd2);
        }
        if (fd1 > 0
            && buf2_avail \- buf2_written > 0) {
            FD_SET(fd1, &wr);
            nfds = max(nfds, fd1);
        }
        if (fd2 > 0
            && buf1_avail \- buf1_written > 0) {
            FD_SET(fd2, &wr);
            nfds = max(nfds, fd2);
        }
        if (fd1 > 0) {
            FD_SET(fd1, &er);
            nfds = max(nfds, fd1);
        }
        if (fd2 > 0) {
            FD_SET(fd2, &er);
            nfds = max(nfds, fd2);
        }

        r = select(nfds + 1, &rd, &wr, &er, NULL);

        if (r == \-1 && errno == EINTR)
            continue;
        if (r < 0) {
            perror("select()");
            exit(EXIT_FAILURE);
        }
        if (FD_ISSET(h, &rd)) {
            unsigned int l;
            struct sockaddr_in client_address;
            memset(&client_address, 0, l = sizeof(client_address));
            r = accept(h, (struct sockaddr *) &client_address, &l);
            if (r < 0) {
                perror("accept()");
            } else {
                SHUT_FD1;
                SHUT_FD2;
                buf1_avail = buf1_written = 0;
                buf2_avail = buf2_written = 0;
                fd1 = r;
                fd2 =
                    connect_socket(forward_port, argv[3]);
                if (fd2 < 0) {
                    SHUT_FD1;
                } else
                    printf("connect from %s\\n",
                            inet_ntoa(client_address.sin_addr));
            }
        }
/* NB: read oob data before normal reads */
        if (fd1 > 0)
            if (FD_ISSET(fd1, &er)) {
                char c;
                errno = 0;
                r = recv(fd1, &c, 1, MSG_OOB);
                if (r < 1) {
                    SHUT_FD1;
                } else
                    send(fd2, &c, 1, MSG_OOB);
            }
        if (fd2 > 0)
            if (FD_ISSET(fd2, &er)) {
                char c;
                errno = 0;
                r = recv(fd2, &c, 1, MSG_OOB);
                if (r < 1) {
                    SHUT_FD1;
                } else
                    send(fd1, &c, 1, MSG_OOB);
            }
        if (fd1 > 0)
            if (FD_ISSET(fd1, &rd)) {
                r =
                    read(fd1, buf1 + buf1_avail,
                          BUF_SIZE \- buf1_avail);
                if (r < 1) {
                    SHUT_FD1;
                } else
                    buf1_avail += r;
            }
        if (fd2 > 0)
            if (FD_ISSET(fd2, &rd)) {
                r =
                    read(fd2, buf2 + buf2_avail,
                          BUF_SIZE \- buf2_avail);
                if (r < 1) {
                    SHUT_FD2;
                } else
                    buf2_avail += r;
            }
        if (fd1 > 0)
            if (FD_ISSET(fd1, &wr)) {
                r =
                    write(fd1, buf2 + buf2_written,
                           buf2_avail \- buf2_written);
                if (r < 1) {
                    SHUT_FD1;
                } else
                    buf2_written += r;
            }
        if (fd2 > 0)
            if (FD_ISSET(fd2, &wr)) {
                r =
                    write(fd2, buf1 + buf1_written,
                           buf1_avail \- buf1_written);
                if (r < 1) {
                    SHUT_FD2;
                } else
                    buf1_written += r;
            }
/* check if write data has caught read data */
        if (buf1_written == buf1_avail)
            buf1_written = buf1_avail = 0;
        if (buf2_written == buf2_avail)
            buf2_written = buf2_avail = 0;
/* one side has closed the connection, keep
   writing to the other side until empty */
        if (fd1 < 0 && buf1_avail \- buf1_written == 0) {
            SHUT_FD2;
        }
        if (fd2 < 0 && buf2_avail \- buf2_written == 0) {
            SHUT_FD1;
        }
    }
    exit(EXIT_SUCCESS);
}
.fi
.PP
The above program properly forwards most kinds of TCP connections
including OOB signal data transmitted by \fBtelnet\fP servers.
It
handles the tricky problem of having data flow in both directions
simultaneously.
You might think it more efficient to use a
.BR fork (2)
call and devote a thread to each stream.
This becomes more tricky than
you might suspect.
Another idea is to set non-blocking I/O using an
.BR ioctl (2)
call.
This also has its problems because you end up having
to have inefficient timeouts.

The program does not handle more than one simultaneous connection at a
time, although it could easily be extended to do this with a linked list
of buffers \(em one for each connection.
At the moment, new
connections cause the current connection to be dropped.
.SH SEE ALSO
.BR accept (2),
.BR connect (2),
.BR ioctl (2),
.BR poll (2),
.BR read (2),
.BR recv (2),
.BR select (2),
.BR send (2),
.BR sigprocmask (2),
.BR write (2),
.BR sigaddset (3),
.BR sigdelset (3),
.BR sigemptyset (3),
.BR sigfillset (3),
.BR sigismember (3),
.BR epoll (7)
.\" .SH AUTHORS
.\" This man page was written by Paul Sheer.